Mobile, transportable electrostatic chucks (Transfer-ESC) are used as mechanical support carriers for thin substrates. This technology is applied for the manufacturing of Integrated Circuits (IC's) within the semiconductor industry. The tendency to thinner chips or wafers is shown on RFID-Chip-products as well as for Power-Chips and many different other applications. Those support carriers allow a safe manipulating of thin, brittle wafers on existing production equipment because the size and thickness of the clamped substrate on the mobile, transportable electrostatic chuck is similar of size, thickness and shape as a standard wafer. An advantage of the support carrier technique is the reversible clamping of the transport substrates outside of the production equipment. Besides, after charging the Transfer-ESC, they do not need additional, external current or voltage power supply for a long period of time. For this reason the package of Transfer-ESC and thin wafer can be handled as a normal thick wafer. The existent transport- and process equipment (grinding-, etch- or polishing equipment, Implanter, PVD-, Sputter- or CVD-tools—see DE 20311625 U1) can be employed further more. After a completed process step, the thin wafer or the separated chips can be taken off form the Transfer-ESC after deactivating the electrostatic clamping force or it can be recharged again. The Transfer-ESC is reusable.
Similar problems as described are found in other industrial branches like the medicine-, solar- or display-industry. As used herein, a “wafer” encompasses any of the various types of substrates that can be held on a chuck, including semiconductor wafers, glass or ceramic plates, or any of various other suitable substrates.
State of the art for 150 μm thin wafers is the use of polymer protection foils for mechanical stabilisation. This support technique seams applicable for a wafer thickness down to 100 μm. The attached/glued protection foils must be peeled off mechanically after later process steps. This can lead to a breakage of the brittle, sensible wafers. The disadvantage of this technique is that the foils are not reusable and they are not resistant against higher temperatures. There application is limited to process steps were the temperature is less than 150 degree Celsius.
Alternatively, Transfer-ESC can be employed as stabilising carriers instead of foils. The connection of a thin wafer and the Transfer-ESC is carried out by applying a clamping voltage (typically 300 V to 3000 V). An electrostatic field will be created between the electrode structure within the Transfer-ESC and the wafer. The resulting clamping force is similar to Coulomb forces of a plate capacitor. After clamping the Transfer-ESC with the attached wafer, it can be transported or processed without any further connection to a voltage or current supply system. After a couple of hours, another recharging of the Transfer-ESC is necessary, because the leakage currents of the capacitor structure (typically <5 nA at room temperature) discharge with time and the clamping forces are reduced. This can lead up to a loss of the clamped wafer.
In EP 1217655 A1a method of manipulating thin wafers is described, which is using the term “Transfer-ESC” the first time for transportable, electrostatic chucks. Within US 2004/0037692 A1 Landesberger et al. describes electrodes, which are arranged in a matrix. By providing the electrode structure in the form of a matrix, individual chips can be removed “pixelwise” by reversing the polarity of respective electrodes of the matrix. Landesberger et al. describes in FIG. 2 a round structure, which consists of quarter-circular-segment electrodes. Each two of the quarter-circular segments are connected to each other and once activated they are on a positive (+) or negative (−) potential. For de-chucking of an earlier separated chip, the appropriate electrode structure is deactivated using the matrix structure and by reversing the polarity of at least two of the quarter-circular-segments the chip can be taken off.
In contradiction to mobile, transportable electrostatic chucks, stationary electrostatic chucks (ESC) are used to clamp wafers in manufacturing tools for the chip industry for decades. Stationary ESC as well as end effectors differentiating in a way that they are permanently connected to a power supply unit and thus they are not mobile. For this reason leakage current in respect of a non sufficient power supply has been of less interest. Those ESC are mostly modified to achieve fast chucking and de-chucking cycle times. Besides, many different designs for electrode structures have been developed. Some examples for unipolar, bipolar and multipolar electrode structures are found in U.S. Pat. No. 4,551,192, U.S. Pat. No. 4,480,284, U.S. Pat. No. 4,184,188, U.S. Pat. No. 4,384,918, U.S. Pat. No. 4,692,836, U.S. Pat. No. 4,724,510, U.S. Pat. No. 5,572,398, U.S. Pat. No. 5,151,845, U.S. Pat. No. 6,174,583, EP 0692814, EP 0460955, EP 1070381. In EP 0880818 B1 a low voltage electrostatic clamp is described. The clamping force is not only depending on the applied voltage but is also significantly influenced by the structure of the electrodes. The formula how to calculate the value of Coulomb forces for unipolar and bipolar chucks is known from analogous considerations of plate capacitors. By using two differently charged long elongated serpentine like allocated electrodes with a width of <100 μm and the spacing between the electrodes of less than 100 μm it was found that clamping forces are improving and were higher than expected. It is argued, that they have created a non-uniform electric field. This non-uniform electric field has an additional force component. Dielectric objects (wafer) can be electrostatically clamped by immersing the object in a non-uniform electric field. The non-uniform electric field produces a force which tends to pull the dielectric object into the region of the highest electric field. For this reason the same clamping force was achieved with a reduced applied clamping voltage. By using a flat-panel-display (AMLCD) manufacturing technology, the smallest width of the electrodes in there array of electrodes was 20 μm and the deposited dielectric layer has had a thickness of 5 μm. The needed clamping voltage was less than 1 kV. State of the art types of stationary ESC use widths of the electrodes of about 3 mm and the spacing between the electrodes is about 1 mm. Typically they work with clamping voltages in the range of 1 kV to 3 kV. The used thickness of the dielectric layer is in the range of 10 μm to 500 μm. Different thick-film technologies are applied to produce stationary ESC. EP 805487 A2 describes the use of fuses to electrically disconnect a failed electrode from the output terminal. This application is related to a stationary ESC which is permanently connected to a power supply unit. The resistor fuses are constructed from resistive materials like nickel-phosphorous, nickel-chromium or others with a length of up to 5 mm. The disadvantage of those types of fuses is that they are not able to be integrated using a suitable thin-film technology.
The proposed solutions do not fulfil further technical and commercial requirements to mobile, transportable electrostatic chucks. Although the risk of breakage of thin (<150 μm) and ultra thin (<50 μm) substrates is drastically reduced by using Transfer-ESC during the manufacturing and transport of wafers, the clamping power is still problematic for some process steps. Those process steps are CVD-, metallization- and anneal processes, which are carried out at temperatures up to 750 degree Celsius. The clamping force of Coulomb chucks is proportional to the square of the applied clamping voltage (U), the dielectric constant (∈r) of the insulator layer and is indirectly proportional to the square of the thickness of the insulator layer (d). Thus, a strong electrostatic holding force is obtained by having a high clamping voltage (U>1000 V) with a high dielectric constant of the material (∈r) is 3.5 to 9) and a very small thickness of the insulation layer. Most of the typical used dielectric materials, as described and cited in the patents above, show a significant decrease of there insulation behavior at elevated temperatures of about 250 degree Celsius. This is causing high leakage currents and thus only a short clamping time. A major drawback is also that a single defect of the dielectric layer can cause a disaster fault of the Transfer-ESC.